Emissions control of sulfur dioxide and nitric oxide in flue gas streams resulting from the combustion of carbon-based fuels in power plants is an advancing art. There is a burgeoning need for an efficient and inexpensive process for abating such pollutant gases to meet ever increasing environmental concerns and the growing need to utilize plentiful, but pollutant-producing fuels, such as coal, to replace diminishing petroleum reserves.
In a widely employed practice, sulfur dioxide-containing flue gases are scrubbed with an aqueous lime or limestone slurry. Sulfur dioxide is removed by precipitation of hydrated CaSO.sub.4 and CaSO.sub.3 salts. A common variation of this process utilizes SO.sub.2 sorbents such as sodium carbonates to produce a bisulfite intermediate. The sorbent may be subsequently regenerated by contacting the bisulfite with lime or limestone. The insoluble calcium salts produced thereby are typically disposed of in landfills.
Metal chelates, and ferrous chelates in particular, have been introduced to address the problem of NO abatement such as, for example, in U.S. Pat. No. 4,732,744 to Chang et al.; No. 4,612,175 to Harkness et al.; No. 4,708,854 to Grinstead; No. 4,615,780 to Walker; and No. 4,126,529 to DeBerry. In these references, SO.sub.2 is scrubbed concurrently by a SO.sub.2 sorbent such as water soluble sulfite, bisulfite and carbonate salts. The SO.sub.2 and NO are typically removed as a soluble sulfur/nitrogen salt waste stream. Separation of these byproduct salts from chelate sorbents also present in the waste steam is accomplished by various techniques such as, for example, in Chang by a crystallization separation process, in Walker and DeBerry by electrodialysis or in Grinstead by ultrafiltration or dialysis.
Related U.S. Pat. Nos. describing the use of chelate sorbents include No. 4,013,430 to Adachi et al.; No. 3,984,522 to Saito et al.; No. 4,044,101 and No. 4,081,509 to Hisamatsu et al.; No. 4,079,118 and No. 4,091,074 and No. 4,079,118 to Gorai; No. 4,158,044 to Takabatake et al.; No. 4,331,639; No. 4,448,899 and No. 4,597,954 to Hass; Nos. 3,991,161; 3,992,508; 4,055,623 and No. 4,087,372 to Saitoh et al.; and No. 3,932,585 to Moriguchi et al. Related articles include S. G. Chang et al., American Chemical Society Symposium Series, No. 188, (1982) and S. A. Bedell et al., Hydrocarbon Processing, Jan. 1988, pp. 63-66.
Electrochemical reduction of oxidized chelate absorbents is disclosed in several references. In DeBerry, the electrochemical cell comprises a series of compartments separated by a series of anionic and cationic membranes. Preferred anode electrodes are said to be made from non-corroding materials handling high current densities such as lead and lead alloys. In Walker, the electrochemical cell is said to comprise a stack of compartments separated by alternating bipolar and anionic selective membranes. Other references disclosing electrochemical reduction of chelates in a SO.sub.2 /NO abatement process include U.S. Pat. No. 4,607,234 to Hoelter; Japanese Patent JA-086017(05.02.76); Chemical Abstracts (CA) 84:169057u (Hasui, et al.); CA 84:169058v (Miki et al.); and CA 90:209418v (Sato et al.). See also, S. A. Bedell et al., Industrial & Chemical Research Vol 26, No. 11, (1988), pp. 2092-2095; Susan S. Tsai et al., Environmental Progress, May 1989, pp 126-129; S. S. Tsai et al., Presentation Paper-American Institute of Chemical Engineers (AICHE), (1987); S. S. Tsai et al., Presentation Paper-AICHE, #81B, (1988); S. S. Tsai et al., Presentation Paper-Air & Waste Management Assoc. Annual Meeting 90-101.4, Jun. 24-29 (1990); and D. J. Zabcik, Presentation Paper-Combined FGD and Dry SO.sub.2 Control Symposium, Oct. 25-28 (1988).
The use of a variety of electrode materials in various different electrochemical processes are described in several references. U.S. Pat. No. 4,814,051 to Bedell discloses platinized titanium or tantalum which have enhanced stability in an electrochemical cell used to regenerate an alkanolamine sorbent for H.sub.2 S. Other U.S. Pat. Nos. 2,768,945 to Shapiro; No. 4,592,814 and No. 4,608,136 to Vaughan et al.; and No. 3,723,264 to Leduc.
It is desirable that a process be devised for the efficient removal of SO.sub.2 and NO from flue gas streams wherein the oxidized chelate absorbents may be reduced, in a simple, two-compartment electrochemical cell wherein the electrodes have high current densities and are stable in the operating environment. In addition, it is desirable that the chelate sorbents be separated from the waste salts for recycle to the absorber for lower operation costs and more efficient operation.